1. Field of the Invention
The present invention relates to a switching amplifier and a signal amplifying method.
2. Description of the Related Art
There are audio amplifiers comprised of switching amplifiers which control switching devices, so as to amplify audio signals.
Conventional switching amplifiers have the following problems to be solved.
A high-performance filter having a complicated structure is needed for cutting off a switching frequency component(s) of a PWM signal, especially, a fundamental wave component(s) having a high energy.
To reproduce audio with high fidelity, it is necessary that the upper limit of the frequency bandwidth of the switching amplifier be set equal to or greater than 100 kHz. To establish that upper limit of the frequency bandwidth, it is necessary to set the switching frequency at or greater than 2 MHz.
However, it is difficult for the switching devices to switch at or grater than 2 MHz in a stable manner. For example, if a regular FET (Field Effect Transistor) is used as a switching device, the regular FET does not have an enough ability of switching at or greater than 2 MHz. Therefore, setting the switching frequency at or greater than 2 MHz causes a delay of the switching and a dispersion of the switching time. Such a delay and the dispersion affect a dead time which is set for preventing a through current flowing through the plurality of switching devices, resulting in large consumption power, and a distortion rate of output of the switching amplifier is aggravated.
The switching frequency may be set at 2 MHz, by decreasing the source voltage of the switching devices. However, if the source voltage is decreased, the amplification factor is decreased as well. As a result of this, the switching amplifier can not produce a large output.
To overcome the above problems that the switching amplifier can not produce a large output, a BTL (Balanced Transformer Less) circuit may be employed in the switching amplifier. The BTL circuit is a full-bridge circuit which amplifies an original signal by a push-pull operation. The BTL circuit has characteristics that a large output can be obtained from the circuit, even if its source voltage is relatively low.
However, even in a switching amplifier using a BTL circuit, it is necessary to set the switching frequency of the switching devices at or greater than 2 MHz. Thus, it is difficult that the switching devices perform switching in a stable manner in the switching amplifier if, for example, a regular FET is used as a switching device.
The present invention has been made in consideration of the above. It is accordingly an object of the present invention to provide a switching amplifier and signal amplifying method, which have lower switching frequency relatively and which reproduce an original signal with high fidelity.
Another object thereof is to provide a switching amplifier and a signal amplifying method, which output an amplified signal with a lower distortion rate relatively.
In order to accomplish the above objects, according to the first aspect of the present invention, there is provided a switching amplifier which generates pulse signals, sets a duty ratio of each of the pulse signals in accordance with a signal level of an original signal to be amplified, switches a direct-current voltage in accordance with each of the pulse signals to generate power-pulse signals, and filters the power-pulse signals, thereby amplifying the original signal and supplies a load with the power-pulse signals.
The amplifier comprises: a pulse-signal generator which generates the pulse signals, each of which has a duty ratio corresponding to the signal level of the original signal and is out of phase with respect to each other; a plurality of amplification sections, which has switching devices, which inputs each of the plurality of pulse signals and performs switching of the direct-current voltage in accordance with the duty ratio of each of the pulse signals, thereby generating a plurality of power-pulse signals which are out of phase; and a filter which combines and filters the plurality of power-pulse signals output from said plurality of amplification sections, and supplies said load with the combined and filtered power-pulse signals.
The filter may include reactors, each of which is coupled to respective one of said plurality of amplification sections for combining the power-pulse signals.
Each of said plurality of amplification sections may include switching devices, which are coupled to a power source applying a direct-current voltage and perform switching of the direct-current voltage. In this case, it is desirable that the pulse-signal generator inverts the plurality of pulse signals, and outputs the plurality of pulse signals and the plurality of inverted signals to said plurality of amplification sections.
Each of said plurality of amplification sections may include switching devices, which are coupled to a power source applying a direct-current voltage and perform switching of the direct-current voltage; and said pulse-signal generator inverts the plurality of pulse signals, and outputs the plurality of pulse signals and the plurality of inverted signals to said plurality of amplification sections.
The switching amplifier may further comprises: a shifting section which shifts a phase of a clock signal by predetermined degrees, so as to generate shifted clock signals; a reference signal generator which generates reference signals from the sifted clock signals which are output from said shifting section; a modulator which compares the original signal to be amplified with the reference signals, thereby modulating the duty ratio of each of the plurality of pulse signals and supplying switching devices of each of the amplification sections with the pulse signals; and an inverter which inverts each of the plurality of pulse signals whose duty ratio has been modulated, and supplies the switching devices of each of the amplification sections with the pulse signals, respectively.
In the switching amplifier, each of the plurality of amplification sections may includes two switching devices, which are arranged in series and accept the direct-current voltage supplied thereto. In this case, a load is coupled to a coupling point of the two serially-arranged switching devices of each of the plurality of amplification sections, through said filter, thereby forming a half or full-bridge circuit; and said pulse-signal generator generates a plurality of pulse signals, which are out of phase with each other, and their inverted signals, and supplies each pair of one of the pulse signals and respective inverted signals to the two serially-arranged switching devices of respective one of said amplification sections.
In the switching amplifier, said pulse-signal generator may include: a comparing-reference signal generator which generates a comparing-reference signal in accordance with phases of the clock signals; a first pulse-signal generator which compares the original signal to be amplified with the comparing-reference signal generated by said comparing-reference signal generator, so as to output a first pulse signal; a first inverted-pulse signal generator which inverts said first pulse signal generated by said first pulse-signal generator, so as to output a first inverted pulse signal; an original-signal inverter which inverts the original signal to be amplified; a second pulse-signal generator which compares the signal, inverted by said original-signal inverter, with the comparing-reference signal generated by said comparing-reference signal generator, so as to output a second pulse signal; a second inverted-pulse signal generator which inverts said second pulse signal generated by said second pulse-signal generator, so as to output a second inverted pulse signal.
The filter may be able to attenuate signal components whose frequencies are higher than a fundamental frequency of the pulse signals.
The filter may combine the power-pulse signals so as to cancel at least a part of a signal component whose frequency is equal to a fundamental frequency of the pulse signal.
The pulse-signal generator may comprise a pulse-width modulator which generates pulse signals, whose phases are shifted from each other and whose pulse widths are modulated in accordance with the signal level of the original signal.
The pulse-signal generator may comprise a frequency modulator which generates pulse signals whose phases are shifted from each other and whose frequencies are modulated in accordance with the signal level of the original signal.
In order to accomplish the above objects, according to the second aspect of the present invention, there is provided a signal amplifying method comprising the steps of: generating a plurality of pulse signals which are out of phase; setting a ratio of an ON-period to an OFF-period of each of the plurality of pulse signals, based on a signal level of an original signal to be amplified; switching a direct-current voltage based on each of the plurality of pulse signals, thereby generating a plurality of power-pulse signals which are out of phase; and combining the plurality of generated power-pulse signals together, filtering the combined power-pulse signals, and supplying a load with the filtered signals, thereby canceling a fundamental wave component and/or a higher harmonic of each of the generated power pulse signals.
In order to accomplish the above objects, according to the third aspect of the present invention, there is provided a switching amplifier comprising: a pulse-signal generator which generates a plurality of pulse signals, whose duty ratios are modulated in accordance with an original signal to be amplified and which are out of phase with each other; a plurality of switching sections which perform a switching operation in accordance with the pulse signals, thereby generating a plurality of power-pulse signals which are out of phase with each other; and a filter which combines and filters the plurality of power-pulse signals to reproduce the original signal, and outputs the reproduced signal.
In order to accomplish the above objects, according to the fourth aspect of the present invention, there is provided a signal amplifying method comprising the steps of: generating a plurality of pulse signals whose duty ratios are modulated in accordance with an original signal to be amplified and which are out of phase with each other; switching in accordance with the pulse signals to generate a plurality of power-pulse signals which are out of phase with each other; and combining and filtering the power-pulse signals to reproduce the original signal, and outputs the reproduced signal.